Study On Diagenetic Response Process And Gas Reservoir Forming Mechanism Of Marine Shale Under The Typical Compressional Tectonic Environment

The marine organic rich shale of the Lower Silurian Longmaxi Formation in the Sichuan Basin,which was characterized by typical "self generation and storage",is an important stratum for marine shale gas exploration and development in China.In recent years,a series of industrial shale gas fields have been formed in Weiyuan,Fuling and other places,which set off an upsurge in shale gas exploration and development.The shale gas exploration practice has confirmed that the reservoir evolution and gas enrichment of shale were comprehensively restricted by various geological processes such as early sedimentary conditions,late structural evolution and diagenesis,which also makes the fine characterization of reservoir and the study of differential enrichment law become the focus of shale gas exploration.However,under the typical compressional tectonic environment,the multi-stage and phased subsidence rates and uplift amplitudes are quite different with different tectonic areas.How does the differential tectonic evolution process control and affect the evolution of shale diagenetic system?How does the differential diagenetic process of shale restrict the material composition,pore structure,reservoir physical and mechanical properties of shale?And what is the difference law of oil and gas generation,migration and shale gas enrichment in shale?At present,there are still insufficient understanding of the above problems,which limits the breakthrough of key problems such as fine characterization of shale reservoir and differential enrichment law of shale gas,and slows down the progress of shale gas exploration and development.Based on the above situation and problems,this paper takes the Lower Silurian Longmaxi Formation shale in different structural areas(low and gentle structural belt in the South Sichuan and high and steep fold belt in Southeast Chongqing)under the typical compressive tectonic environment as an example.Sedimentary environment,organic geochemistry,diagenetic evolution process,reservoir evolution,enrichment and accumulation characteristics were compared with the help of nano analysis methods such as argon ion polishing and focused ion beam scanning electron microscope,This paper mainly revealed the evolution track of shale diagenetic system,reservoir evolution law and gas reservoir enrichment model under different sedimentary and structural evolution conditions,in order to provide new examples for the study of shale diagenetic effect and differential enrichment mechanism of gas reservoir under special structural background.At the same time,the following innovative understanding has been obtained.(1)The diagenetic differential evolution and response process of organic rich shale in different structural areas are revealed.The Lower Silurian Longmaxi Formation shale in the whole Sichuan Basin experienced seven diagenetic evolution stages due to deep burial process:syngenetic rock,early diagenetic rock A,early diagenetic rock B,middle diagenetic rock A,middle diagenetic rock B,late diagenetic rock and post diagenetic rock.Compaction,clay mineral transformation,cementation,dissolution and hydrocarbon generation of organic matter were developed,and the inorganic-organic synergistic diagenetic process was very complex.The diagenetic process of the Longmaxi Formation shale in the low and gentle structural belt in the South Sichuan mainly occurred in a relatively closed diagenetic environment,with complete clay minerals transformation,obvious dissolution of soluble minerals,and sufficient thermal evolution of organic matter.Clay minerals mainly consist of illite and chlorite.Organic matter pores occur as long strips.inorganic pores and organic matter nano pores were also developed.The diagenetic process of the Longmaxi Formation shale at the edge of the basin(Southeast Chongqing)mainly occurred in a relatively open diagenetic environment.The clay minerals mainly comprise chlorite and illite mixed layers.The mixed layer minerals are beneficial to improve the specific surface area and adsorption capacity of shale,and the organic matter pores are mainly round and honeycomb.(2)The differential forming mechanism of shale gas reservoir under compressive tectonic environment was clarified.The deep-water shelf sedimentary environment of Early Silurian in the Sichuan Basin provided good enrichment and preservation conditions for the Longmaxi Formation shale organic matter,so that Longmaxi Formation shale has strong hydrocarbon generation potential.the Longmaxi Formation shale,which was characterized by complete clay minerals transformation((%S<10)),sufficient thermal evolution of organic matter(Ro>2.0%),a large number of connected organic matter pores and matrix pores due to the diagenetic evolution,provided good nano reservoir conditions for shale gas in the low and gentle structural belt in the South Sichuan.Although the Longmaxi Formation shale in Sichuan Basin generally has good roof and floor conditions,the efficient enrichment time of shale gas was shortened drastically by early and greatly uplift of the basin edge(Southeast Chongqing)since the late Cretaceous.The existence of celestial fault and exposed surface destroyed the formation pressure system and preservation conditions of shale gas was basically destroyed and lost because of celestial fault and exposed surface exposure.It is not conducive to the effective enrichment and accumulation of shale gas,and there is the possibility of effective enrichment and accumulation only in areas with weak local structural transformation(Jiaoshiba and other areas).It is suggested that the"hindcasting" be used for the optimization of favorable target areas for marine shale gas exploration in South China.The "gold/platinum" can be selected only after the fine comparative study of geochemical and reservoir conditions is carried out in the areas with preservation conditions.